UNIVERSITY     OF    CALIFORNIA 

COLLEGE   OF    AGRICULTURE 

AGRICULTURAL    EXPERIMENT    STATION 

BERKELEY,    CALIFORNIA 

CIRCULAR  No.  283 

April,  1925 

FERTILIZING  CITRUS  TREES 
IN  CALIFORNIA 

By  KOBERT  W.  HODGSON 


INTRODUCTION 

The  annual  fertilizer  bill  of  the  California  citrus  fruit  growers 
reaches  a  total  of  several  million  dollars.  Surveys  indicate  that 
expenditures  for  fertilizers  constitute  the  largest  item  in  the  average 
citrus  orchard  management  programme,  amounting  to  about  sixty  dol- 
lars per  acre  per  year.  They  are  in  many  cases  higher,  not  infrequently 
amounting  to  as  much  as  one  hundred  and  fifty  dollars.  These  expendi- 
tures are  relatively  large  as  compared  to  the  practice  in  other  fruit 
growing  industries  and  indicate  that  the  citrus  fruit  growers  of  this 
state  are  convinced  of  the  necessity  and  importance  of  fertilization  as 
an  aid  to  profitable  production.  It  is  believed  that  there  is  ample 
evidence  available,  however,  to  warrant  the  statement  that  by  exercis- 
ing more  discrimination  in  the  selection  of  fertilizers  in  accordance 
with  our  present  knowledge  of  their  effects  on  citrus  trees,  the  same 
general  results  might  be  had  from  much  smaller  expenditures.  It  is 
certain  that  many  citrus  growers  purchase  large  amounts  of  fertilizers 
which,  so  far  as  our  present  knowledge  goes,  do  not  pay  returns  on 
the  investment.  It  is  equally  certain  that  by  a  greater  use  of  certain 
fertilizers,  many  growers  might  materially  increase  their  orchard 
yields  with  profit. 

For  some  eighteen  years  the  California  Agricultural  Experiment 
Station  has  been  conducting  fertilizer  tests  with  citrus  trees.*  Among 
the  most  important  of  these  have  been  the  trials  at  the  Rubidoux  site 
of  the  Citrus  Experiment  Station,  the  Arlington  tests,  and  fertilizer 


*  The  list  of  workers  contributing  materially  to  our  knowledge  of  citrus 
fertilization  in  California  includes  the  following:  Ralph  E.  Smith,  J.  W.  Mills, 
T.  Francis  Hunt,  J.  H.  Norton,  J.  Eliot  Coit,  H.  J.  Webber,  W.  M.  Mertz,  W.  P. 
Kelley,  K.  S.  Vaile,  C.  J.  Booth,  Gordon  Surr,  and  J.  A.  Prizer. 


2  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

experiments  at  Ontario,  Chula  Vista,  and  Naranjo.  A  number  of 
surveys  relating  to  fertilizer  practice  have  also  been  made.  It  is  the 
purpose  of  this  circular  to  present  a  brief  summary  of  our  present 
knowledge  of  this  important  subject,  and  recommendations  based  on 
the  work  done  by  this  station.  The  data  used  in  compiling  the  present 
publication  have  already  been  presented  in  bulletins,  technical  papers, 
and  addresses  before  groups  of  citrus  growers.* 

The  form  of  presentation  followed  will  be  the  answering  of  the 
questions  most  commonly  raised  by  the  growers  which  for  convenience, 
are  set  forth  as  follows  : 

1.  Do  citrus  trees  require  fertilization? 

2.  What  plant  food  elements  and  other  materials  supplied  by 
fertilizers  produce  measurable  increases  in  yield  or  improvements  in 
the  quality  of  the  fruit  ? 

3.  How  much  nitrogen  per  tree  gives  the  best  results? 

4.  What  forms  of  nitrogen  fertilizers  give  the  best  results  ? 

5.  How  much  of  the  nitrogen  should  be  supplied  in  bulky  organic 
form? 

6.  What  are  the  comparative  availabilities  of  the  different  nitrogen 
fertilizers  and  of  what  practical  importance  are  these  differences  ? 

7.  When  should  the  different  nitrogen  fertilizers  be  applied  for  the 
best  results  ? 

8.  What  methods  of  application  are  recommended? 

9.  What  is  the  fertilizer  value  of  a  covercrop  ? 

10.  What  are  the  values  of  lime,  gypsum,  sulfur,  and  other  soil 
amendments  ? 

11.  What  is  the  value  of  peat? 

12.  How  should  fertilizers  be  selected  and  purchased  ? 

1.  In  California  citrus  trees  require  fertilization  if  crop  production 
is  to  be  maintained. 
In  the  early  years  of  the  citrus  industry  in  California  the  only 
guide  available  to  growers  was  experience  and  practice  elsewhere.  It 
was  known  that  the  use  of  considerable  quantities  of  fertilizers  was  the 
practice  in  Florida  and  in  the  citrus  districts  of  the  old  world.  The 
natural  assumption,  therefore,  was  that  in  all  probability  the  citrus 
orchards  in  California  would  require  fertilization.  Within  a  few  years 
experience  was  accumulated  substantiating  this  assumption.  It  was 
commonly  observed  that  the  fertilized  orchards  generally  were  more 
productive  than  those  not  fertilized.     It  was  further  noted  that  it 


*  A  list  of  the  more  important  papers  and  bulletins  dealing  with  this  subject 
will  be  found  at  the  end  of  this  paper. 


CIRC.  283]  FERTILIZING    CITRUS    TREES   IN    CALIFORNIA  3 

frequently  required  a  period  of  eight  or  ten  years  before  these  differ- 
ences became  evident,  but  this  was  readily  explainable  on  the  basis  of 
the  high  initial  fertility  of  the  soil  in  many  of  the  older  citrus  districts. 

Although  quantitative  data  in  sufficient  amount  to  establish  the 
necessity  for  fertilization  in  the  commercial  production  of  citrus  fruits 
was  lacking,  by  1900  this  requirement  was  generally  accepted  by 
growers  and  investigators.  Since  that  time,  however,  ample  evidence 
to  establish  this  point  beyond  question  has  been  accumulated. 

In  1907  a  survey  of  fertilizer  practice  in  some  fifty  citrus  orchards 
in  southern  California  furnished  consistent  data  showing  that  the  fer- 
tilized orchards  were  higher  in  yield  than  the  unfertilized  orchards. 
In  1913  a  report  on  the  Rubidoux  trials,  established  in  1907,  called 
attention  to  the  fact  that  unfertilized  trees  at  six  and  seven  years  of 
age  had  already  failed  to  produce  satisfactory  crops.  From  that  time 
on  the  differences  became  much  more  marked  in  the  Rubidoux  plots 
and  after  reaching  ten  years  of  age  as  reported  in  1922,  the  unfertilized 
trees  bore  practically  no  fruit.  That  the  necessity  for  fertilizing  citrus 
trees  had  been  fully  demonstrated  is  one  of  the  conclusions  set  forth 
in  the  general  summary  of  citrus  fertilizer  trials  in  California  pub- 
lished in  1922.  It  may  therefore  be  said  without  qualification  that  for 
the  economical  and  profitable  production  of  citrus  fruits  in  California, 
fertilization  is  required. 

2.  Nitrogen  is  the  only  element  and  organic  matter  the  only  other 

material  which   have   been   demonstrated   to  give   measurable 

improvements  in  yield  and  tree  health.     The  application  of 

phosphorus  or  potassium  has  not  been  shown  to  be  necessary  or 

beneficial. 

Experience  developed   in  the  older   agricultural   sections  of  the 

world,  and  particularly  in  the  humid  regions  has  led  to  the  conclusion 

that  nitrogen,  phosphorus  and  potassium  are  the  elements  most  likely 

to  be  exhausted  in  cropped  soils.    The  assumption  of  the  necessity  of 

adding  these  in  order  to  maintain  crop  production  has  furnished  the 

basis   for   fertilizer   practice   the   world   over.     Attention  was  early 

directed  to  nitrogen  as  the  probable  limiting  factor  in  crop  production 

on  California  soils,  extensive  studies  indicating  that  this  element  is  of 

primary  importance  and  that  organic  matter  is  likewise  an  essential 

in  maintaining  crop  production.    The  conclusion  was  also  reached  that 

phosphorus  is  the  element  of  second  importance  and  that  only  rarely 

is  potassium  needed. 

In  order  to  gain  more  exact  information  as  to  the  different  elements 
required  in  the  fertilization  of  citrus  trees  and  also  to  compare  differ- 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

ent  sources  of  these  elements,  the  Citrus  Experiment  Station  in  1907 
established  at  Riverside  the  Rubidoux  fertilizer  experiment,  already 
referred  to.  This  experiment  has  contributed  much  to  our  knowledge 
of  citrus  fertilization.  In  the  original  experiment  there  were  20  plots, 
to  which  two  more  were  added  later.  The  three  plant  food  nutrients 
were  applied  separately  and  in  various  combinations,  and  the  experi- 
ment was  arranged  in  such  a.  manner  as  to  make  possible  comparisons 
not  only  between  the  different  elements  but  also  between  various 
carriers  of  these  elements.  The  carriers  of  nitrogen  were  given  especial 
attention. 

Nitrogen. — Although  for  ten  or  fifteen  years  before  the  establish- 
ment of  the  Rubidoux  experiment  it  had  been  felt  that  high  yields 
were  correlated  with  the  use  of  nitrogenous  fertilizers  or  fertilizers 
high  in  nitrogen  content,  specific  data  on  this  point  were  not  forth- 
coming until  the  results  of  the  Rubidoux  experiment  began  to  develop. 
Within  a  few  years  after  the  experiment  was  established,  outstanding 
effects  from  the  use  of  nitrogen  fertilizers  became  apparent. 

The  first  report  on  this  experiment,  made  in  1913,  states  that  at 
that  time,  when  the  trees  were  six  and  seven  years  old,  the  only 
measurable  increases  in  production  had  been  secured  from  the  nitrogen 
plots.  In  1914,  the  results  of  cooperative  fertilizer  tests  maintained 
at  Santa  Paula,  Riverside  and  Pomona,  were  reported  with  the  state- 
ment that  the  tests  showed  unmistakable  results  from  the  use  of  nitro- 
genous fertilizers.  In  1915,  a  fertilizer  practice  survey  conducted  in 
the  Ontario-Upland  district  revealed  a  strong  correlation  between 
high  yields  and  the  use  of  relatively  large  amounts  of  nitrogen.  The 
1920  report  on  the  results  of  the  Arlington  experiment,  established  by 
the  Citrus  Experiment  Station  in  1915,  states  that  the  plots  receiving 
commercial  fertilizers  high  in  nitrogen  had  shown  a  marked  response 
in  increased  production  the  first  season  of  application  and  these  plots 
had  continued  to  show  measurable  increases  in  yield  during  the  five- 
year  period  of  the  experiment.  The  1922  summary  of  results  on  the 
Rubidoux  plots  states  that  the  trials  there  had  demonstrated  con- 
clusively that  nitrogen  is  the  main  limiting  plant  food  element. 

In  the  summary  of  all  citrus  fertilizer  tests,  in  the  same  report,  the 
general  conclusion  is  advanced  that  in  all  cases  where  it  had  been 
possible  to  make  comparisons  or  to  analyze  the  results,  measurable 
increases  in  production  were  always  associated  with  the  use  of  nitrogen. 

Organic  matter. — The  fundamental  importance  of  organic  matter 
in  the  soil  has  of  course  been  recognized  almost  from  the  beginnings  of 
agriculture,  and  from  ancient  times  tillage  and  cropping  practices 


CIRC.  283]  FERTILIZING    CITRUS    TREES   IN    CALIFORNIA  5 

designed  to  assist  in  the  maintenance  of  a  high  content  of  organic 
matter  have  been  recognized  as  an  essential  feature  of  good  farm  prac- 
tice. The  importance  of  organic  matter  in  California  soils  was  early 
emphasized  and  it  has  long  been  known  that  in  addition  to  its  nitrogen 
content  there  are  other  important  reasons  for  its  beneficial  effects  in 
the  soil. 

While  the  importance  of  organic  matter  was  long  ago  recognized  by 
the  citrus  growers  and  large  quantities  have  been  used  for  many  years, 
it  was  not  until  comparatively  recently  that  quantitative  data  were 
secured  demonstrating  its  necessity  and  importance  in  citrus  fertiliza- 
tion. As  early  as  1900,  the  Experiment  Station  recommended  the  use 
of  large  quantities  of  organic  matter  in  the  fertilization  of  citrus  trees. 
Quantitative  data  bearing  on  this  question  did  not  become  available, 
however,  until  supplied  by  the  Rubidoux  tests.  Observations  made 
by  various  members  of  the  Experiment  Station  show  that  from  prac- 
tically the  beginning,  in  1907,  the  plots  on  which  manure  was  applied, 
either  with  or  without  a  winter  covercrop,  were  equal  to  those  receiving 
concentrated  nitrogenous  fertilizers.  Within  a  few  years  it  became 
apparent  that  the  plots  receiving  organic  matter  were  superior  to 
certain  of  the  concentrated  nitrogen-fertilized  plots  and  this  superior- 
ity continued  to  develop  until  by  1920,  by  general  agreement,  they 
were  regarded  as  clearly  superior  to  all  other  plots  in  the  experiment. 
In  1922  these  plots  still  maintained  their  superiority  as  shown  in  the 
report  published  in  that  year. 

Reporting  on  the  effects  of  organic  matter  furnished  by  covercrops, 
in  1918  data  were  submitted  showing  striking  increases  in  yield  and 
much  better  health  of  trees  on  the  covercropped  plots  than  on  the  plots 
fertilized  with  manure  alone.  The  1920  report  on  the  Arlington  tests 
indicated  that  the  trees  on  the  plots  receiving  only  manure,  in  com- 
parison to  those  receiving  large  quantities  of  nitrogen  from  chemical 
sources,  were  in  much  the  best  condition  after  five  years,  although 
second  in  yield.  Since  that  time,  although  additional  applications 
were  not  made,  the  manure  plots  have  consistently  ranked  first  in  both 
yield  and  tree  condition. 

In  the  summary  of  all  the  citrus  fertilizer  tests  published  in  1922, 
the  conclusion  is  advanced  that  bulky  organic  matter  has  always  shown 
pronounced  beneficial  results,  when  furnished  either  in  the  form  of 
manure  alone  or  with  the  addition  of  the  winter  covercrop.  Striking 
evidence  on  this  point  has  recently  been  brought  out  in  a  survey  of 
six  hundred  citrus  fruit  orchards,  which  will  be  referred  to  later  under 
another  heading.  It  is  therefore  clear  that  the  substantial  benefit  from 
the  use  of  decomposable  organic  matter  in  citrus  fertilization  has  been 


6  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

well  established  on  the  basis  of  quantitative  data  accumulated  during 
many  years.  The  cumulative  effects  of  its  use  serve  to  emphasize  the 
importance  of  annual  applications  of  this  kind  of  material. 

Phosphorus  and  Potassium. — The  so-called  ready-mixed  complete 
fertilizers  containing  the  three  plant  nutrients,  nitrogen,  phosphorus, 
and  potassium,  came  into  prominence  early  in  the  history  of  the  citrus 
industry  in  California,  and  were  used  extensively,  especially  after 
agreement  had  been  reached  to  the  effect  that  fertilization  was  a 
profitable  practice.  The  proponents  of  the  use  of  these  fertilizers, 
however,  while  usually  willing  to  admit  the  importance  of  nitrogen, 
almost  from  the  beginning  have  claimed  special  benefits  from  their  use. 
Among  these  claims  have  been  improvements  in  eating  and  keeping 
quality  of  the  fruit,  the  former  principally  by  increase  in  sugar  con- 
tent and  the  production  of  a  smoother  and  thinner  rind,  and  the 
latter  by  imparting  unusual  powers  of  resistance  to  decay  producing 
organisms.  The  advocates  of  the  use  of  potassium  have  been  especially 
active  in  advancing  claims  of  these  benefits  and  in  addition  have 
always  held  that  this  element  materially  increases  the  resistance  of 
citrus  trees  to  frost  injury.  Claims  of  increased  yields  attendant 
upon  the  use  of  these  elements  have  also  been  advanced,  though  not  so 
much  in  recent  years  as  formerly.  The  necessity  of  a  "balanced 
ration"  for  citrus  trees  is  also  an  argument  much  used  by  the  pro- 
ponents for  the  complete  fertilizer. 

While  thousands  of  tons  of  ready -mixed  complete  fertilizers  have 
been  purchased  by  the  citrus  growers  on  the  basis  of  these  claims, 
analysis  of  the  fertilizer  tests  on  citrus  trees  in  this  state  furnishes 
little  evidence  to  support  the  arguments  of  benefits  from  the  applica- 
tion of  phosphorus  or  potassium,  singly,  together,  or  in  combination 
with  nitrogen. 

Although  applications  of  phosphorus  and  potassium  had  been  made 
for  six  years,  in  1913  no  measurable  improvements  from  the  use  of 
these  elements  had  been  noted  in  the  Rubidoux  plots.  The  1914  report 
on  the  cooperative  field  trials  at  Santa  Paula,  Riverside  and  Pomona, 
previously  referred  to,  states  that  they  furnished  no  evidence  indicat- 
ing benefits  from  the  use  of  phosphorus  or  potassium.  In  one  experi- 
ment where  potassium  alone  was  tested,  the  results  were  reported  as 
entirely  negative.  In  the  summary  of  the  Rubidoux  plots  made  in 
1922,  it  is  stated  that  the  plots  receiving  phosphorus  or  potassium  or 
both,  but  no  nitrogen,  had  shown  no  significant  differences  in  yield  or 
other  characters  from  the  unfertilized  plots,  and  advanced  the  con- 
clusion that  no  evidence  whatever  had  been  secured  to  show  that  the 
use  of  either  of  these  elements  had  increased  yield  or  improved  quality. 


CIRC.  283]  FERTILIZING    CITRUS    TREES   IN    CALIFORNIA  7 

The  summary  of  the  Arlington  fertilizer  plots  given  in  the  same  report, 
states  that  since  the  best  fruit  had  been  produced  on  plots  receiving 
nitrogen  only,  there  was  no  evidence  that  phosphorus  or  potassium  are 
essential  for  the  production  of  citrus  fruits  of  good  quality.  The 
general  summary  of  all  citrus  fertilizer  work  done  in  California,  issued 
in  1922,  concludes  that  no  positive  evidence  of  benefits  from  the  use 
of  these  two  elements  has  yet  been  shown.  Additional  evidence  to 
support  these  general  conclusions  has  been  secured  from  the  Chaffey 
fertilizer  plots.  The  1923  report  on  these  plots  states  that  as  yet  there 
is  no  evidence  of  increases  of  yield  from  the  use  of  these  elements  and 
that  the  best  fruit  in  the  experiments  has  been  produced  on  plots  not 
receiving  either. 

So  far  as  composition  of  the  fruit  is  concerned,  it  is  interesting  to 
note  that  the  only  measurable  effect  thus  far  noted  has  been  a  slight 
influence  of  nitrogen  in  the  Rubidoux  trials,  which  was  reported  in 
1917  to  have  resulted  in  a  slightly  lower  amount  of  sugar,  a  somewhat 
coarser  fruit,  and  a  little  less  juice.  These  differences,  however,  had 
in  no  case  resulted  in  lowering  the  commercial  grade  of  the  fruit. 

It  is  readily  admitted,  however,  that  the  data  available  concerning 
the  specific  effects  of  the  different  plant  nutrients  upon  the  fruit,  and 
especially  upon  its  keeping  and  shipping  quality,  are  meager.  More 
data  are  required  before  final  conclusions  on  these  points  can  be  drawn 
with  certainty.  Further  studies  of  these  questions  are  now  being 
outlined. 

While  there  is  no  doubt  that  citrus  trees  require  phosphorus  and 
potassium,  it  appears  that  these  are  either  already  available  in  suffi- 
cient amounts  in  California  soils,  or  are  made  available  as  needed  by 
the  decomposition  of  organic  materials.  Moreover,  it  should  be  pointed 
out  that  the  use  of  animal  manures  provides  relatively  large  amounts 
of  these  two  elements.  There  is  certainly  as  yet  no  experimental  evi- 
dence whatever  that  phosphorus  and  potassium  can  take  the  place  of 
organic  matter  or  that  applications  of  these  elements  have  proved 
necessary  or  beneficial. 

3.  From  two  to  three  pounds  of  nitrogen  per  tree  per  year  is  ordinarily 
the  most  profitable  amount  of  this  element,  although  larger 
amounts  may  produce  more  fruit. 
With  a  growing  recognition  of  nitrogen  as  the  principal  plant 
nutrient  required  in  citrus  fertilization,  the  question  of  how  much 
nitrogen  should  be  used  to  maintain  the  trees  in  good  health  and  pro- 
duction becomes  one  of  importance,  as  is  also  the  inquiry  as  to  the 
amount  of  nitrogen  it  is  profitable  to  apply.    Wide  variation  exists  in 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

the  amount  of  nitrogen  used.  When  the  ready-mixed  complete  fertil- 
izers were  used  extensively,  the  average  amount  of  nitrogen  furnished 
was  about  one  pound  per  tree.  As  it  became  apparent  that  this  element 
was  responsible  'for  the  increased  production,  in  many  cases  the 
amounts  used  were  increased  materially,  although  usually  without 
reference  to  supplying  any  definite  quantity  per  tree.  In  a  general 
way  the  range  in  the  use  of  this  element  has  been  from  1  to  5  pounds 
per  tree. 

Measurable  increases  in  yield  have  frequently  been  noted  from  the 
use  of  relatively  small  amounts  of  nitrogen,  from  0.5  to  1  pound  per 
tree,  and  one  pound  per  tree  has  been  the  basis  of  much  citrus  fertilizer 
practice.  Data  from  tests  bearing  on  this  point,  however,  are  com- 
paratively meager.  The  nitrogen  plots  in  the  Rubidoux  experiment, 
were  fertilized  at  the  rate  of  1.28  pounds  per  tree.  The  nitrogen  plots 
in  the  Arlington  experiment  received  approximately  1.5  pounds  per 
tree.  In  1921  tests  were  established  at  Ontario  in  which  1,  2,  and  3 
pounds  per  tree  were  compared,  and  at  about  the  same  time  the  farm 
advisors  in  several  counties  established  similar  trials. 

In  1922  and  1923  an  extensive  survey  on  the  use  of  nitrogen  was 
conducted,  in  which  the  yields  of  six  hundred  citrus  orchards  for  a 
period  of  five  years  were  analyzed  on  the  basis  of  the  amount  of  nitro- 
gen applied  annually  during  that  period  as  correlated  with  yield.  The 
results  of  this  survey  constitute  by  far  the  most  comprehensive  data 
available  bearing  on  this  point,  and  show  that  there  is  a  regular  increase 
in  yield  where  amounts  of  nitrogen  are  used  up  to  350  pounds  per  acre, 
approximately  4  pounds  per  tree.  It  should  be  emphasized,  however, 
that  an  unhealthy  condition  of  the  trees  was  shown  to  be  correlated 
with  the  use  of  this  quantity  of  nitrogen.  The  general  conclusion  was 
reached  that  the  most  profitable  amount  of  this  element  to  use  lies 
betwen  250  and  300  pounds  per  acre  when  the  price  received  for  the 
fruit  averages  two  cents  per  pound  or  better.  When  lower  prices 
prevailed  the  most  profitable  use  of  nitrogen  appeared  to  be  from  150 
to  250  pounds  per  acre. 

In  1924  data  were  reported  showing  that  two  pounds  per  tree  had 
been  the  most  profitable  use  of  this  element  in  the  Chaffey  experi- 
ments, although  three  pounds  was  the  most  productive  use.  It  is 
believed  that  the  evidence  now  available  is  sufficient  to  indicate  rather 
definitely  that  about  200  pounds  per  acre  of  nitrogen  is  the  most  profit- 
able amount  for  young  bearing  trees  and  about  300  pounds  for  the 
older  trees.  How  much  of  the  different  nitrogenous  fertilizers  is 
required  to  supply  these  amounts  is  shown  in  Table  1. 


Circ.  283] 


FERTILIZING    CITRUS    TREES   IN    CALIFORNIA 


9 


4.  No  one  source  of  nitrogen  has  been  shown  to  be  best  but  the  evidence 

is  conclusive  that  part  of  it  should  be  supplied  in  bulky  organic 

form. 

Ever  since  the  importance  of  nitrogen  has  been  recognized,  citrus 

fruit  growers  have  held  individual  preferences  as  to  sources  of  this 

element.    Kather  definite  beliefs  have  existed  that  certain  nitrogenous 

fertilizers  are  better  than  others,  although  there  has  never  been  any 

TABLE   1 

Nitrogen  Content  of  Different  Fertilizers  and  Amounts  Required  to  Supply 
200  Pounds  of  Nitrogen  per  Acre 


Nitrogenous  fertilizers 


Alfalfa  hay 

Bean  straw 

Ordinary  manures 

High  grade  manures 

Nitrate  of  soda 

Nitrate  of  lime 

Sulfate  of  ammonia.. 

Bone  meal 

Low  grade  tankage... 

Garbage  tankage 

High  grade  tankage. 

Cottonseed  meal 

Fish  meal 

Dried  blood 


Average 

nitrogen 

content  in 

percent 


2.0 
1.5 
0.5 
1.5 


15.0 

12.0 

20.0 

3.0 

5.0 

2.5 

9.0 

6.0 

10.0 

12.0 


Amount  required  to  supply 
200  lbs. 


Per  acre 


5      tons 

6.6  tons 

20.0  tons 

6.6  tons 


1333  lbs. 
1667  lbs. 
1000  lbs. 
6667  lbs. 
4000  lbs. 
8000  lbs. 
2222  lbs. 
3333  lbs. 
2000  lbs. 
1667  lbs. 


Per  tree* 


125  lbs. 

167  lbs. 

500  lbs. 

167  lbs. 


16. 6  lbs. 
20.8  lbs. 

12.5  lbs. 
83.3  lbs. 
50.0  lbs. 

100.0  lbs. 

27.7  lbs. 

41.6  lbs. 
25.0  lbs. 

20.8  lbs. 


For  converting  tons  into  cubic  measure  a  value  of  35  to  40  pounds  per  cubic 
foot  for  clean  and  relatively  dry  manure  may  be  used. 

*  On  basis  of  eighty  trees  to  the  acre. 


general  agreement  as  to  which  is  best.  The  evidence  available  at  the 
present  time  indicates  that  nitrogen  from  one  source  is  quite  as  effec- 
tive as  that  from  any  other,  provided  the  total  amounts  supplied  are 
the  same  and  the  rates  of  decomposition  and  methods  of  application 
such  as  to  make  it  available  to  the  trees  in  equal  quantities  at  the  same 
time.  Study  of  the  various  tests  thus  far  conducted  indicates  the 
impossibility  of  deriving  any  general  conclusions  with  reference  to  the 
best  source  of  nitrogen.  In  respect  to  yield,  health  of  trees,  and  quality 
of  fruit,  the  best  results  within  the  different  individual  fertilizer  tests 
have  frequently  been  secured  by  nitrogen  from  different  sources. 


10  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Thus,  the  general  summary  of  citrus  fertilizer  results  published 
in  1922,  concludes  that  no  source  of  nitrogen  has  been  found  to  be  the 
best  and,  comparing  the  plots  in  which  nitrogen  was  applied  in 
chemical  form,  in  the  form  of  high  grade  organic  fertilizers,  and  in 
complete  fertilizers,  states  that  the  results  have  been  similar.  Again, 
the  1923  report  on  the  Chaff ey  plots  indicates  fluctuations  back  and 
forth  in  reference  to  the  source  of  nitrogen  which  gave  the  best  results. 

The  evidence  that  at  least  a  part  of  the  nitrogen  used  can  be  sup- 
plied with  advantage  from  bulky  organic  sources  is  overwhelming,  as 
indicated  previously  in  the  discussion  of  the  necessity  of  organic 
matter.  Thus  the  superior  condition  of  the  manure-covercrop  plots  at 
Kubidoux  has  been  recognized  for  many  years  and  would  certainly 
indicate  that  a  part,  if  not  all,  of  the  nitrogen  should  be  in  bulky 
organic  form. 

The  same  conclusion  is  reached  in  the  general  summary  of  fertilizer 
work  with  citrus  trees,  namely,  that  bulky  organic  matter  has  always 
shown  beneficial  results,  manure  being  always  to  be  regarded  as  a 
good  source  of  a  large  part  of  the  nitrogen,  and  alfalfa  hay  and  bean 
straw  being  also  excellent  sources  of  this  element. 

5.  The  evidence  available  at  this  time  indicates  that  at  least  half  of  the 
nitrogen  should  be  supplied  from  bulky  organic  sources.  With 
bearing  trees  there  is  evidence  to  indicate  that  a  part  of  the 
nitrogen  can  be  supplied  in  concentrated  form  with  advantage. 
By  far  the  most  comprehensive  data  bearing  on  the  proportion  of 
the  nitrogen  used  which  can  be  supplied  with  advantage  from  bulky 
organic  sources,  are  furnished  by  the  extensive  survey,  already  referred 
to,  on  citrus  orchard  fertilizer  practices.  In  this  survey  a  positive 
correlation  was  established  between  yield  and  amount  of  nitrogen 
applied  in  the  form  of  manure.  It  was  shown,  on  the  basis  of  the 
practice  followed  by  many  hundreds  of  growers,  that  high  yields  are 
definitely  associated  with  the  use  of  large  amounts  of  manure,  up  to 
20  tons  per  acre.  Arbitrarily  setting  a  figure  of  100  as  the  average 
yield  of  the  orchards  in  which  60  per  cent  or  more  of  the  nitrogen  was 
applied  in  bulky  organic  form,  it  was  found  that  in  the  orchards  where 
this  source  supplied  only  from  40  to  60  per  cent  the  relative  yield  was 
93,  and  in  orchards  where  less  than  40  per  cent  of  the  nitrogen  came 
from  bulky  sources,  89.  On  the  basis  of  tons  of  manure  applied  per 
acre,  it  was  shown  that  the  relative  yield  of  orchards  where  less  than 
five  tons  were  applied  was  86,  as  compared  to  100  for  the  highest 
yielding  class  where  from  fifteen  to  twenty  tons  were  used. 

The  observation  made  by  many  growers  as  well  as  by  Station 
workers  that  applications  of  quickly  available  nitrogen  in  the  spring 


ClRC.  283]  FERTILIZING    CITRUS   TREES   IN    CALIFORNIA  11 

frequently  occasion  a  heavier  set  of  fruit,  is  supported  by  the  positive 
results  on  this  point  in  the  Arlington  experiment.  This  indicates,  that 
while  bulky  organic  sources  alone  are  much  superior  to  chemical 
sources  or  to  organic  concentrates  alone,  it  is  probable  that  a  combina- 
tion of  the  two  will  be  found  most  advantageous.  Thus  the  1922  sum- 
mary states  that  it  would  appear  that  the  combination  of  manure  or 
covercrop  with  some  high  grade  source  of  nitrogen  is  better  than  either 
alone.  Corroboration  of  these  results  has  also  been  furnished  in  the 
five-year  soil  management  tests  in  Tulare  County,  conducted  by  the 
Citrus  Experiment  Station. 

The  conclusion  seems  warranted  therefore  that  for  the  bearing 
orchard,  at  least  one-half  the  nitrogen  used  should  be  furnished  by 
bulky  organic  materials  and  the  rest  by  quickly  available  concentrates. 

6.  The  nitrogenous  fertilizers  may  be  divided  into  two  groups — those 
quickly  available,  and  those  slowly  available.  Choice  between 
these  groups  depends  principally  upon  time  of  application. 

Availability  may  be  defined  as  the  rate  at  which  the  plant  nutrients 
in  a  soil  or  fertilizer  are  released  and  converted  into  forms  in  which 
plants  can  use  them.  With  the  nitrogenous  fertilizers,  this  form  is 
believed  to  be  chiefly  nitrates.  The  importance  of  availability  is  related 
primarily  to  the  time  of  application  of  the  fertilizers.  Thus  a  quickly 
available  fertilizer  should  be  applied  close  to  the  period  when  it  is 
required  by  the  plant,  and  conversely,  a  slowly  available  fertilizer 
should  be  applied  some  time  before  it  is  required,  in  order  that  it  may 
have  sufficient  time  to  pass  through  the  processes  which  render  it 
available.  While  there  are  wide  variations  in  availability  in  the 
nitrogenous  fertilizers,  studies  made  by  the  Experiment  Station  in 
recent  years  indicate  that  these  gradations  are  not  so  marked  as  was 
formerly  believed  and  that  it  is  possible  to  establish  two  general 
classes — the  quickly  available  sorts,  and  those  more  slowly  available. 

From* the  standpoint  of  nitrogen  content  a  further  classification  of 
the  nitrogenous  fertilizers  into  two  principal  groups,  the  bulky  organic 
materials  and  the  concentrates,  is  convenient.  The  concentrates  are 
further  divided  according  to  source,  into  the  inorganic  (chemical) 
forms  and  organic  forms.  Investigations  have  shown  that  in  general 
the  bulky  organics  are  slowly  available  although  there  are  several 
exceptions,  including  alfalfa  hay,  green  manure  crops  (covercrops), 
and  poultry  manure,  which  are  to  be  regarded  as  fairly  quickly  avail- 
able. Bean  straw  is  perhaps  the  slowest  of  the  bulky  organics  to 
become  available  and  barnyard  manure  intermediate  in  position. 
Barley  straw  has  been  shown  to  be  not  only  extremely  slowly  available 
but  for  long  periods  specifically  depressive  to  the  nitrifying  bacteria 


12  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

in  the  soil.  The  use  of  this  material  therefore  is  advised  against,  except 
for  the  improvement  of  the  physical  condition  of  very  heavy  soils. 
Manures  high  in  cereal  straw  content  may  therefore  be  regarded  as  in 
general  very  slowly  available  and  sometimes  depressive  to  nitrification. 

Although  slight  differences  exist,  the  concentrates  are  all  fairly 
quickly  available.  Two  of  the  chemical  forms  now  on  the  market, 
nitrate  of  soda  and  nitrate  of  lime,  being  already  in  the  nitrate  form, 
are  immediately  available  and  do  not  require  transformation  in  the 
soil.  The  others  must  be  nitrified  in  the  soil  and  this  will  take  more 
or  less  time  up  to  several  weeks,  according  largely  to  the  temperature 
and  moisture  conditions. 

Nitrate  of  Soda. — Especial  reference  should  be  made  to  the  use  of 
nitrate  of  soda  as  a  source  of  quickly  available  nitrogen  for  citrus 
trees.  The  markedly  injurious  effects  noted  in  the  Rubidoux  plots 
where  nitrate  of  soda  alone  has  been  used  as  a  source  of  nitrogen  have 
been  regarded  as  at  least  reflecting  suspicion  on  this  fertilizer.  It  is 
by  no  means  certain,  however,  that  other  chemical  sources  of  nitrogen 
might  not  have  produced  equally  unfavorable  results  if  they  had  been 
used  in  a  similar  manner.  On  fairly  open,  porous  soils  where  the 
drainage  is  good  so  as  to  prevent  the  accumulation  of  the  sodium 
residue  from  this  fertilizer,  there  appears  to  be  little  reason  for  antici- 
pating injurious  effects  from  its  use,  especially  if  applied  in  reasonable 
amounts  in  conjunction  with  large  quantities  of  organic  materials. 

A  general  classification  of  the  nitrogenous  fertilizers  according  to 
content  of  nitrogen  and  availability  is  as  follows: 

(1)  Bulky  organic  (0.5  to  2.5  per  cent  nitrogen). 

fBean  straw 

I  Animal  manures 
Slowly  available  <-p. 

J  I  Dry  cover  crops 

[Garbage  tankage 

f  Alfalfa  hay 

Quickly  available  J  Poultry  manure 

[Green  manure  crops 

(2)  Concentrates  (2  to  20  per  cent  nitrogen). 
All  quickly  available 

fNitrate  of  soda 
Inorganic  (chemical)  J  Nitrate  of  lime 

Sulfate  of  ammonia 
f  Dried  blood 

Cottonseed  meal 
'l  High  and  low  grade  tankages 
Fish  scrap  or  meal 
Whale  meat 
Bone  meal 


Organic  < 


ClRC  283]  FERTILIZING    CITRUS    TREKS    IX    CALIFORNIA  13 

7.  In  general  the  bulky  organic  materials,  with  the  exception  of  the 

quickly  available  forms,  should  be  applied  in  the  fall.  The 
concentrates,  on  the  other  hand,  should  be  applied  in  the  spring. 

As  was  brought  out  in  the  discussion  concerning  availability,  the 
slowly  available  fertilizers  should  be  applied  some  time  in  advance 
of  the  period  when  the  nutrients  they  contain  are  needed.  In  the  case 
of  bean  straw  this  may  and  usually  does  require  three  or  four  months, 
with  barnyard  manure  perhaps  a  slightly  shorter  time,  and  with  alfalfa 
hay  a  few  weeks.  The  question  of  when  to  apply  the  nitrogenous 
fertilizers  is  dependent,  therefore,  upon  a  knowledge  of  the  period 
when  needs  of  the  plants  for  nitrogen  are  greatest.  While  undoubtedly 
citrus  trees  require  some  nitrogen  throughout  the  entire  year,  there 
appears  to  be  a  fairly  definite  period  when  this  nutrient  is  of  special 
importance.  With  orchard  fruits  in  general  this  is  believed  to  be  just 
before  and  during  the  blooming  and  fruit-setting  period.  It  is  of 
special  interest  to  note  that  at  this  period  the  amount  of  soil  nitrates 
in  citrus  orchards  is  frequently  comparatively  low. 

There  is  ample  observational  evidence  as  well  as  considerable 
experimental  evidence  -to  indicate  that  with  citrus  trees  the  most 
marked  response  to  the  use  of  nitrogenous  fertilizers  is  secured  when 
these  are  applied  so  as  to  furnish  available  nitrogen  during  the  bloom- 
ing and  fruit-setting  period.  The  Arlington  experiments  and 
numerous  others  have  furnished  positive  evidence  in  support  of  this 
conclusion. 

In  general  therefore,  the  quickly  available  fertilizers  should  be 
applied  in  the  spring,  preferably  at  the  time  of  plowing  if  this  is  not 
delayed  until  too  close  to  the  period  of  bloom,  and  for  the  best  results 
the  more  slowly  available  forms  should  be  applied  in  the  fall. 

8.  Deep  application  where  possible,  preferably  by  plowing,  is  recom- 

mended. 
It  cannot  be  said  that  any  one  method  of  applying  fertilizers  to 
citrus  trees  has  been  shown  to  be  superior  to  all  others.  Whatever  the 
method  used,  the  main  object  should  be  to  place  the  fertilizers  where 
the  feeder  roots  can  utilize  them.  Avoidance  of  losses  of  organic 
matter  from  excessive  oxidation  incident  to  cultivation  and  of  nitrates 
from  leaching,  are  also  factors  of  importance  in  determining  the 
method  of  application.  Since  the  feeder  roots  are  located  below  the 
surface  mulch,  it  is  important  that  the  method  of  application  be  such 
as  to  insure  the  placement  of  fertilizers  below  that  zone.  To  accomp- 
lish this  with  the  bulky  organic  materials  requires  deep  incorporation, 
and  this  should  preferably  be  done  by  plowing  deep.  If  surface  appli- 
cations of  these  materials  are  to  be  made,  the  best  season  for  so  doing 


14  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

is  the  winter  months,  when  a  minimum  amount  of  cultivation  is  prac- 
ticed and  oxidation  losses  of  organic  matter  therefore  lightest.  This 
practice  is  not  recommended  for  the  heavy  soils  in  districts  where 
spring  rains  are  normally  heavy,  however,  on  account  of  the  tendency 
to  keep  the  soil  wet  and  cold  which  materially  depresses  nitrate  forma- 
tion. Reasons  of  convenience  frequently  require  the  application  of 
organic  materials  at  other  seasons  of  the  year  and  where  this  is  neces- 
sary it  is  important  that  they  be  immediately  incorporated  as  deeply 
in  the  soil  as  it  is  feasible  under  the  circumstances. 

With  reference  to  the  application  of  organic  materials  in  deep 
furrows  or  trenches,  it  appears  that  this  method  is  especially  beneficial 
for  young  trees  and  in  older  orchards  where  the  tillage  and  fertilizer 
practice  has  been  such  as  to  encourage  surface  feeding.  In  the  case 
of  young  trees,  each  year  the  furrows  should  be  moved  farther  out  so 
as  to  overcome  any  tendency  toward  restriction  in  root  distribution. 
In  the  older  orchards  this  method  is  suggested  only  as  an  occasional 
practice  to  supplement  surface  distribution  and  deep  incorporation  of 
these  materials. 

In  a  similar  way  the  concentrates  should  preferably  be  placed  deep 
in  the  soil,  with  the  possible  exception  of  late  winter  or  early  spring 
applications  of  the  chemical  forms  which  are  readily  carried  down  by 
rain.  Experience  indicates  that  the  application  of  concentrates  on 
the  surface  or  in  shallow  furrows  is  a  wasteful  practice  since  under  the 
usual  furrow  irrigation  method  employed  the  nitrates  are  rather 
quickly  concentrated  in  the  surface  mulch.  Therefore  plowing  down 
such  fertilizers  or  applying  them  in  deep  irrigation  furrows  is 
recommended. 

9.  The  winter  cover  crop  has  a  pronounced  fertilizer  value  in  the  young 
citrus  orchard  and  in  the  older  orchard,  a  replacement  value  of 
three  to  five  tons  of  manure. 

So  far  as  the  value  of  the  covercrop  is  concerned,  the  principal 
data  available  have  to  do  with  the  leguminous  winter  covercrop.  Under 
favorable  conditions  the  late  summer  covercrop  may  have  a  positive 
fertilizer  value,  especially  in  the  young  orchard,  but  in  the  bearing 
orchard  factors  entering  into  its  management  are  so  complex  that  it  is 
doubtful  whether  they  can  be  satisfactorily  controlled.  Experience 
and  observation  have  indicated  that,  under  many  conditions  at  least, 
the  growing  of  a  summer  covercrop  in  the  citrus  orchard  is  a  practice 
of  doubtful  value. 

In  the  young  orchard  the  fertilizer  value  of  the  winter  covercrop 
is  unquestionably  high  as  has  been  brought  out  strikingly  in  work  done 


CIRC.  283]  FERTILIZING    CITRUS    TREES   IN    CALIFORNIA  15 

at  the  Citrus  Experiment  Station  and  reported  on  in  1918.  This  work 
indicates  conclusively  that  in  the  young  orchard  the  covercrop  is  a 
means  of  greatly  stimulating  growth  and  production.  The  pronounced 
value  of  covercrops  has  also  been  shown  in  the  young  orchard  at  the 
new  Experiment  Station  site.  The  trees  were  planted  in  1917  and  at 
the  present  time,  eight  years  later,  this  orchard  is  generally  regarded 
as  considerably  above  the  average  for  its  age.  No  fertilizers  have  been 
applied  and  its  present  condition  is  believed  to  be  largely  due  to  the 
effects  of  the  winter  and  summer  covercrops  grown. 

In  the  bearing  citrus  orchard  the  covercrop  is  unquestionably  of 
considerable  value  where  a  reasonable  tonnage  can  be  grown  without 
competition  with  the  trees.  The  data  from  the  tests  on  older  orchards, 
however,  are  conflicting,  the  probable  explanation  of  wdiich  is  com- 
petition with  the  trees  either  for  nitrogen  or  water. 

The  orchard  practice  survey  previously  referred  to  brings  out 
clearly  the  fact  that  in  the  bearing  orchard  the  covercrop  has  a  definite 
organic-matter  replacement  value.  Where  the  smallest  amounts  of 
manure  were  used,  it  was  shown  that  the  covercrop  gave  the  most 
striking  results,  with  an  average  increase  in  yield  of  about  20  per  cent. 
Where  the  largest  amounts  of  manure  were  used,  the  least  effect  from 
the  covercrop  was  noted  in  an  increased  yield  of  6  per  cent.  A  good 
covercrop  consisting  of  12  to  15  tons  of  green  material  should  furnish 
approximately  two  tons  of  dry  matter,  each  of  which  is  equivalent  in 
fertilizer  value  (nitrogen  and  organic  matter)  to  two  tons  of  ordinary 
manure.  We  do  not  know  how  much  of  the  nitrogen  in  the  covercrop 
is  taken  directly  from  the  soil  nor  is  it  known  how  much  nitrogen  is 
added  to  the  soil  by  the  bacteria  in  the  nodules  on  the  roots  of  the 
covercrop.  The  relative  proportions  of  these  two  undoubtedly  vary 
considerably.  On  the  basis  of  the  general  considerations  presented, 
however,  it  is  believed  that  there  is  justification  for  estimating  the 
covercrop  replacement  value  at  from  three  to  five  tons  of  manure. 

10.  The  use  of  lime,  gypsum,  sulfur,  and  other  soil  amendments  is 
rarely  productive  of  measurable  benefits. 
The  evidence  with  reference  to  the  use  of  soil  amendments  is  frag- 
mentary and  in  some  cases  conflicting.  In  most  of  the  tests,  however, 
these  materials  have  given  negative  results.  In  one  or  two  tests  there 
have  been  slight  indications  of  benefits  from  the  application  of  lime 
and  gypsum.  The  1922  summary  of  results  of  fertilizer  tests  with 
citrus  trees  states  that  five  years  of  lime  applications  on  the  Arlington 
plots  had  shown  no  benefits  whatever.  The  same  conclusion  has  been 
reached  by  a  number  of  the  farm  advisors  from  tests  conducted  in 
their  counties. 


16  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  citrus  tree  has  a  high  calcium  content,  however,  as  shown  by 
studies  at  the  Citrus  Experiment  Station,  and  recent  investigations 
indicate  that  the  absence  of  this  element  from  the  soil  solution  quickly 
brings  about  a  condition  of  disease.  On  the  other  hand  we  know  that 
there  are  practically  no  acid  soils  in  California  citrus  districts  and 
from  hundreds  of  analyses,  it  has  been  shown  that  citrus  soils  are  in 
general  quite  high  in  calcium.  In  addition  there  is  considerable  evi- 
dence to  show  that  the  decomposition  of  organic  matter  in  the  soil  has 
much  to  do  with  making  calcium  available.  At  the  present  time  we  can 
only  conclude  that  the  use  of  lime  is  rarely  productive  of  measurable 
improvements. 

The  same  may  be  said  with  reference  to  gypsum,  sulfur,  and  other 
soil  amendments.  Where  the  citrus  grower  wishes  to  use  these  mate- 
rials it  is  recommended  that  they  be  applied  on  a  few  rows  only,  with 
untreated  trees  left  to  afford  a  measure  of  possible  benefits. 

11.  Peat  cannot  be  regarded  as  a  satisfactory  manure  substitute  since 
little  or  none  of  its  nitrogen  is  available  and  its  organic  matter 
is  not  readily  decomposed. 

The  occurrence  of  extensive  peat  deposits  adjacent  to  the  citrus 
districts  in  many  parts  of  California  has  been  responsible  for  recurrent 
exploitations  of  this  material  as  a  fertilizer  for  citrus  trees.  On 
account  of  its  similarity  in  composition  to  barnyard  manure  the  tend- 
ency has  been  to  exploit  it  on  the  basis  of  a  manure  substitute, 
especially  when  inoculated  with  various  bacterial  cultures. 

Studies  made  by  the  Experiment  Station  during  recent  years, 
however,  have  shown  rather  conclusively  that  the  available  nitrogen 
content  of  peat  is  very  small  and  that  the  organic  material  in  this 
substance  is  decomposed  with  great  difficulty  if  at  all,  in  California 
soils.  Experiments  in  which  peat  has  been  applied  in  amounts  suffi- 
cient to  supply  two  hundred  pounds  of  nitrogen  per  acre  have  failed 
to  show  significent  improvements  in  yield  or  other  characteristics. 

It  would  appear,  therefore,  that  whatever  benefit  if  any,  is  to  be 
derived  from  the  use  of  this  material  must  be  altogether  from  its 
effect  on  the  physical  condition  of  the  soil,  and  obviously  the  amounts 
required  to  produce  such  an  influence  are  so  great  as  to  render  its  use 
impracticable  in  nearly  all  cases.  If  peat  could  be  obtained  at  no 
cost,  it  is  doubtful  whether  it  would  be  worth  hauling  any  great 
distance  to  the  orchard. 

In  any  event  it  is  certain  that  peat,  treated  or  otherwise,  cannot  be 
regarded  as  a  fertilizer  for  citrus  trees,  and  by  no  means  as  a  substi- 
tute for  manure  or  other  sources  of  decomposable  organic  matter. 


Cmc.  283] 


FERTILIZING    CITRUS    TREES    IN    CALIFORNIA 


17 


12.  Fertilizers  should  be  purchased  on  the  basis  of  their  unit  costs  for 
the  plant  nutrients  desired.     In  the  case  of  the  concentrates, 
nitrogen  is    the   effective    constituent;   in   the    bulky   organic 
materials,  nitrogen  and  organic  matter. 
A  number  of  factors  may  enter  into  the  choice  of  fertilizers,  among 
which  are  the  materials  available  on  the  market,  the  time  when  the 
fertilizers  are  to  be  applied,  the  fertilizer  programme  followed,  the 
market  price  per  unit  for  the  plant  nutrients,  and  others.    As  already 
indicated,  it  is  believed  that  on  bearing  trees  at  least  half  of  the  nitro- 
gen should  be  applied  in  bulky  organic  form,  and  the  remainder  in  the 
form  of  concentrates.     With  the  latter,  other  things  being  equal,  a 
choice  of  materials  should  be  based  on  the  comparative  cost  per  unit 
of  nitrogen,  and  in  the  case  of  the  former,  the  cost  of  the  organic 
matter  in  addition. 

The  concentrates  are  now  generally  sold  on  the  unit  basis,  at  prices 
which  vary  widely  according  to  demand  and  supply.  Prices  are  gen- 
erally quoted  on  the  unit  basis,  the  unit  being  1  per  cent  of  a  ton  or 
20  pounds.  With  some  of  these  materials,  however,  the  practice  of 
quoting  them  on  the  cost  per  ton  is  still  followed.  Where  this  is  done 
the  cost  per  unit  should  be  computed  by  dividing  the  ton  cost  by 'the 
nitrogen  content  in  per  cent.  On  account  of  the  wide  variation  in 
prices  for  these  materials,  by  the  exercise  of  a  little  discrimination  in 
choice,  the  opportunity  is  afforded  for  effecting  material  saving  in 

TABLE  2 

Variation  in  Costs  of  Fertilizers,  per  Ton,  per  Pound,   and  on   Basis   of 

Amounts  Eequired  to  Yield  iy±  Pounds  of  Nitrogen  per  Tree 

(100  Pounds  per  Acre) 


Fertilizers 


Ordinary  manures 

High  grade  manures... 

Bean  straw 

Alfalfa  hay 

Nitrate  of  soda 

Nitrate  of  lime 

Sulfate  of  ammonia 

High  grade  tankage... 

Cottonseed  meal 

Fish  meal 

Dried  blood 

*Asof  November  1,  1924 


Nitrogen 
content 

in 
percent 

Price 
per  ton* 

Price 

per  pound 

in  cents 

Pounds  to 
supply 

lj  pounds 
nitrogen 

0.5 

$4.50 

0.225 

250.0 

1.5 

13.50 

0.675 

83.3 

1.5 

16.00 

0.8 

83.3 

2.0 

30.00 

1.5 

62.5 

15.0 

55.00 

2.75 

8.3 

12.0 

60.00 

3.0 

10.4 

20.0 

67.00 

3.35 

6.2 

9.0 

53.00 

2.65 

13.8 

6.0 

47.50 

2.37 

20.8 

10.0 

53.00 

2.65 

12.5 

12.0 

65.00 

3.25 

10.4 

Cost  (in 
cents)  of 
amount 
to  yield  l\ 
pounds 
nitrogen 

56.25 
56.23 
66.64 
93.75 
22.83 
31.20 
20.77 
36.57 
49.10 
33.13 
33.80 


18 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  3 

Variation  in  Costs  per  Tree  for  Fertilizer  Combinations  Yielding  2%  Pounds 

of  Nitrogen,*  One-jialf  from  Bulky  Organic  Sources,  the 

Eemainder  from  Concentrates 


Bulky  organics 

Concentrates 

Ordinary- 
manures 

High  grade 
manures 

Bean 

straw 

Alfalfa 
hay 

Nitrate  of  soda 

$0.79 
.88 
.77 
.93 
1.05 
.89 
.90 

$0.79 
.87 
.77 
.93 
1.05 
.89 
.90 

$0.89 

.98 

.87 

1.03 

1.16 

.99 

1.00 

$1.17 

Nitrate  of  lime 

1.25 

Sulfate  of  ammonia.... 

1.15 

High  grade  tankage 

1.30 

Cottonseed  meal 

1.41 

Fish  meal 

1.27 

Dried  blood 

1.28 

*  At  the  rate  of  200  pounds  per  acre. 

costs.  An  idea  of  the  extent  of  the  variation  in  cost  which  normally 
occurs,  and  of  the  possibility  of  economizing  in  the  purchase  of 
fertilizers  may  be  gained  from  tables  2  and  3. 

Other  things  being  equal,  the  cheapest  nitrogen  and  organic  matter 
should  be  purchased,  provided  of  course  that  the  latter  is  of  a  char- 
acter readily  decomposable  in  the  soil.  Organic  matter  of  a  leguminous 
source  such  as  manure  derived  largely  from  nitrogenous  feeds  (alfalfa 
hay,  cottonseed  cake,  etc.),  alfalfa  hay,  or  bean  straw,  are  usually 
more  desirable  than  organic  matter  high  in  cellulose  (cereal  straws, 
etc.).  Of  the  concentrates,  the  chemical  carriers  are  usually  the 
cheapest.  Thus,  if  cottonseed  meal,  with  an  analysis  of  6  per  cent 
nitrogen  is  quoted  at  $42  per  ton,  the  cost  per  unit  for  this  element  will 
be  $7  or  35  cents  per  pound.  If  sulfate  of  ammonia,  analyzing  20 
per  cent,  is  quoted  at  $70  per  ton,  the  cost  per  unit  will  be  $3.50,  or 
just  half  that  of  the  cottonseed  meal.  In  this  case  therefore  it  would 
certainly  be  economical  to  use  the  chemical  source  for  that  part  of  the 
total  nitrogen  to  be  supplied  from  concentrates. 

Since  the  bulky  organic  fertilizers  are  valuable  chiefly  for  nitrogen 
and  organic  matter,  their  cost  should  be  calculated  on  this  basis  and 
this  practice  is  rapidly  coming  into  use.  The  organic  matter  in  such 
materials  is  usually  quoted  at  prices  ranging  from  four  to  six  cents 
per  unit.  Thus  bean  straw  analyzing  at  1%  per  cent  nitrogen  and  80 
per  cent  organic  matter  would  be  worth  for  nitrogen  at  $5  per  unit 
$7.50,  and  for  organic  matter  at  5  cents  per  unit  $4,  or  a  total  of 
$11.50  per  ton. 


CIRC.  283]  FERTILIZING    CITRUS    TREES   IN    CALIFORNIA  19 


EXPERIENCE  IN  ORCHARD  FRUIT   FERTILIZATION 

During  the  past  twenty  years,  the  fertilization  of  fruit  trees  has 
received  much  attention  at  the  hands  of  experiment  station  workers 
in  various  parts  of  the  United  States  from  the  Pacific  coast  to  the 
Atlantic  seaboard.  While  the  results  in  some  cases  have  been  con- 
flictory,  in  the  words  of  a  recent  worker,  summarizing  the  various 
experiments,  " there  is,  however,  general  agreement  in  some  respects: 
potash  applications  have  almost  without  exception  resulted  in  no 
increase,  wherever  tried ;  phosphorus  applications  have  been  almost  as 
uniformly  unproductive.  The  one  element  that  has  stood  out  con- 
spicuously, where  fertilization  has  proved  profitable,  is  nitrogen." 
In  the  cases  where  potassium  and  phosphorus  have  been  reported  as 
beneficial,  almost  without  exception  it  has  been  in  connection  with 
the  growth  of  covercrops.  Observations  of  the  effects  of  these  elements 
on  the  growth  of  covercrops  in  citrus  orchards  in  California  as  yet 
have  failed  to  furnish  indications  of  substantial  benefits,  the  results 
being  negative  in  nearly  all  cases. 


THE  SOIL  MANAGEMENT  PROGRAMME  AND  FERTILIZATION 

Experience  in  California  and  elsewhere  indicates  a  close  relation 
between  the  soil  management  programme  and  the  effects  obtained 
from  the  application  of  fertilizers.  The  results  of  tests  in  other  states 
have  led  some  investigators  to  the  conclusion  that  the  system  of  soil 
management  used  is  of  more  importance  than  the  fertilizer  programme 
followed.  While  it  is  possible  that  this  may  be  true  under  certain  con- 
ditions the  evidence  with  reference  to  citrus  trees  in  California  does 
not  fully  support  this  conclusion. 

It  is  certain,  however,  that  the  soil  management  programme  may 
overcome  or  at  least  partially  mask  the  effects  of  good  fertilizer  prac- 
tice. This  has  been  brought  out  in  a  striking  manner  in  a  five-year 
test  of  soil  management  practices  in  citrus  orchards  in  Central  Cali- 
fornia where  immediate  and  marked  differences  in  yield  and  tree 
conditions  were  shown  even  though  the  plant  nutrients  and  amounts 
of  water  applied  were  practically  the  same.  It  is  clearly  evident  from 
this  experiment  as  well  as  from  observation  and  experience  that  the 
soil  management  programme  should  be  outlined  and  conducted  so  as 
to  supplement  the  fertilizer  practice  followed,  as  it  is  only  where  this  is 
done  that  maximum  results  from  expenditures  for  fertilizers  are 
obtained. 


20  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Among  the  soil  management  practices  believed  to  materially  affect 
the  efficiency  of  the  fertilizer  programme  are  time  and  depth  of  plow- 
ing down  the  covercrop  in  the  spring,  and  the  irrigation  practice, 
especially  in  the  spring  and  early  summer  months.  The  irrigation 
and  cultural  practices  should  be  such  that  the  soil  warms  up  early 
in  the  spring  and  the  roots  are  amply  supplied  with  available  nitrates. 
This  can  be  accomplished  only  by  a  proper  combination  between  the 
soil  management  programme  and  the  fertilizer  practice,  and  without 
this  combination  the  maximum  results  from  the  fertilizer  programme 
cannot  be  expected. 


SUMMARY 

1.  In  California,  citrus  trees  require  fertilization  if  crop  production 
is, to  be  maintained. 

2.  Nitrogen  is  the  only  element  and  organic  matter  the  only  other 
material  which  have  been  demonstrated  to  give  measurable  improve- 
ments in  yield  and  tree  health. 

3.  The  application  of  phosphorus  or  potassium  has  not  been  shown 
to  be  necessary  or  beneficial. 

4.  From  two  to  three  pounds  of  nitrogen  per  tree  per  year  is 
ordinarily  the  most  profitable  use  of  this  element,  although  larger 
amounts  may  produce  more  fruit. 

5.  No  one  source  of  nitrogen  has  been  shown  to  be  best  but  the 
evidence  is  conclusive  that  part  of  it  should  be  supplied  in  bulky 
organic  form. 

6.  The  evidence  available  at  this  time  indicates  that  at  least  half 
of  the  nitrogen  should  be  supplied  from  bulky  organic  sources. 

7.  With  bearing  trees  there  is  evidence  to  indicate  that  a  part  of 
the  nitrogen  can  be  supplied  with  advantage  in  concentrated  form. 

8.  The  nitrogenous  fertilizers  may  be  divided  into  two  groups — 
those  quickly  available  to  the  trees,  and  those  slowly  available.  Choice 
between  these  groups  depends  principally  upon  the  time  of  application. 

9.  In  general  the  bulky  organic  materials,  with  the  exception  of  the 
quickly  available  forms,  should  be  applied  in  the  fall. 

10.  The  concentrates,  on  the  other  hand,  should  be  applied  in  the 
spring. 

11.  Deep  application  where  possible,  preferably  by  plowing,  is 
recommended. 


CIRC.  283]  FERTILIZING    CITRUS    TREES   IN    CALIFORNIA  21 

12.  The  winter  covercrop  has  a  pronounced  fertilizer  value  in  the 
young  citrus  orchard  and  in  the  older  orchard,  a  replacement  value  of 
three  to  five  tons  of  manure. 

13.  The  use  of  lime,  gypsum,  sulfur,  and  other  soil  amendments  is 
rarely  productive  of  measurable  benefits. 

14.  Peat  cannot  be  regarded  as  a  satisfactory  manure  substitute 
since  little  or  none  of  its  nitrogen  is  available  and  its  organic  matter 
is  not  readily  decomposed. 

15.  Fertilizers  should  be  purchased  on  the  basis  of  their  unit  cost 
for  the  plant  nutrients  desired.  In  the  case  of  the  concentrates,  nitro- 
gen is  the  effective  constituent ;  in  the  bulky  organic  materials,  nitrogen 
and  organic  matter. 

16.  As  reported  by  recognized  authorities,  the  general  fertilizer 
experience  with  orchard  fruits  in  the  United  States  is  similar  to  that 
with  citrus  fruits  in  California,  measurable  improvements  being 
associated  almost  invariably  with  "the  use  of  organic  matter  and 
nitrogen. 

17.  The  soil  management  and  fertilizer  programmes  should  sup- 
plement each  other  if  maximum  benefits  are  to  be  derived  from 
expenditures  for  fertilizers. 


THE  FERTILIZES  PROGRAMME  IN  THE  CITRUS  ORCHARD 

1.  In  the  young  orchard  grow  covercrops  (winter  always,  late 
summer  if  possible)  but  keep  them  under  control.  This  treatment 
alone  is  usually  sufficient  for  the  first  seven  or  eight  years. 

2.  For  young  bearing  trees  grow  a  winter  covercrop  and  apply 
bulky  organic  materials  to  supply  a  total  of  two  pounds  of  nitrogen  per 
tree  per  year.  This  treatment  should  suffice  until  the  trees  are  12 
to  15  years  old. 

3.  From  this  time  on  apply  two  to  three  pounds  of  nitrogen  per 
tree,  approximately  half  of  which  should  be  from  bulky  organic  sources 
and  the  remainder  from  concentrates. 

4.  Use  lime  and  other  soil  amendments  only  where  trials  show 
definite  measurable  improvement  in  tree  health  or  yield. 


22  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


LIST  OF  THE  MORE  IMPORTANT  PUBLICATIONS  DEALING 

WITH  FERTILIZER  RESULTS  APPLICABLE  TO 

CITRUS  TREES  IN  CALIFORNIA 

Alderman,  W.  H. 

1922.  The  present  status  of  orchard  fertilization.     Kansas  State  Hort.  Soe. 

Biennial  Eeport  36:  126-129. 
Booth,  C.  J. 

1923.  Eeport  on  Chaff ey  fertilizer  experiments.     California  Citrograph  8: 

pp.  387,  397. 
Burd,  J.  S. 

1918.  Peat  as  a  manure  substitute.     California  Agr.  Exp.  Sta.   Circ.  203: 

1-10. 
Kelley,  W.  P. 

1917.  Nitrification  in  California  soils.  California  Citrograph  2:  no.  8, 
pp.  7,  14,  15. 

1917.  The  fertilization  of  citrus.     California  Agr.  Exp.  Sta.  Circ.  171:  1-4. 
1922.     Our  present  knowledge  concerning  fertilization  of  citrus.     California 

Citrograph  7:  pp.  186,  202. 
Lipman,  C.  B.,  and  Wank,  M.  E. 

1924.  The  availability  of  nitrogen  in  peat.     Soil  Sci.  18:  311-316. 
McBeth,  I.  G.,  and  Allison,  J.  E. 

1919.  Necessity  for  manure  standardization.     California  Citrograph  4:  259, 

278,  279. 
Mertz,  W.  M. 

1918.  The   use   of  green  manure   crops  in   southern   California.      California 

Agr.  Exp.  Sta.  Bull.  292:  1-31. 
Vaile,  E.  S. 

1914.  Eeview  of  practical  experiments  in  fertilization.  Proc.  45th  Calif. 
Fruit  Growers '  Convention,  Los  Angeles,  Nov.  10-14,  1914,  pp. 
135-136.  Also  in  Monthly  Bull.  California  State  Com.  Hort.  3: 
501-503. 

1921.  Progress   report   on   Arlington   grove    experiment.      California   Citro- 

graph 6:  44,  66,  67.     Dec.  1920.     Also  in  Second  Ann.  Eeport  Cali- 
fornia Citrus  Institute,  pp.  8-16. 

1922.  Fertilizer  experiments  with  citrus  trees.     California  Agr.  Exp.  Sta. 

Bull.  345:  465-512. 
1924.     A   survey   of   orchard   practices   in   the   citrus   industry   of   southern 

California.     California  Agr.  Exp.  Sta.  Bull.  374:  4-40. 
Webber,  H.  J. 

1913.     Fertilizer  experiments  with  citrus  fruits.     California  Cultivator  41: 

596. 
1914-1915.     The  fertilizer  requirements   of  citrus   trees.     Proc.  45th   Calif. 

Fruit   Growers'   Convention,   Los   Angeles,   Nov.   10-14,   1914,  pp. 

101-106.     Also  in  Monthly  Bull.   California  State  Com.  Hort.  4: 

225-230.     1915. 
Young,  H.  D. 

1917.     Effect  of  fertilizers  on  the  composition  and  quality  of  oranges.    Jour. 

Agr.  Eesearch  8:  127-138. 


STATION  PUBLICATIONS  AVAILABLE  FOR  FREE  DISTRIBUTION 


BULLETINS 
No.  No. 

253.   Irrigation   and   Soil   Conditions  in  the  352. 

Sierra  Nevada  Foothills,  California. 

261.  Melaxuma    of    the    Walnut,     "Juglans  353. 

regia."  354. 

262.  Citrus   Diseases   of   Florida   and   Cuba  357. 

Compared  with  Those  of  California. 

263.  Size  Grades  for  Ripe  Olives. 

268.   Growing  and  Grafting  Olive  Seedlings.  358. 

273.    Preliminary  Report  on  Kearney  Vine- 
yard Experimental  Drain.  359. 

275.  The  Cultivation  of  Belladonna  in  Cali-  361. 

fornia. 

276.  The  Pomegranate.  362. 

277.  Sudan  Grass  363. 

278.  Grain   Sorghums. 

279.  Irrigation  of  Rice  in  California.  364. 

280.  Irrigation  of  Alfalfa  in  the  Sacramento 

Valley.  366. 

283.   The  Olive  Insects  of  California. 

285.  The  Milk  Goat  in  California.  367. 

286.  Commercial  Fertilizers. 

294.   Bean   Culture  in  California.  368. 

304.  A  Study  of  the  Effects  of  Freezes  on 

Citrus   in   California.  369. 

310.   Plum  Pollination.  370. 

312.  Mariout  Barley.  371. 

313.  Pruning  Young  Deciduous  Fruit  Trees. 

319.    Caprifigs  and  Caprification.  372. 

324.  Storage  of  Perishable  Fruit  at  Freezing 

Temperatures.  374. 

325.  Rice  Irrigation  Measurements  and  Ex- 

periments    in     Sacramento     Valley, 
1914-1919.  375. 

328.   Prune  Growing  in  California. 

331.   Phylloxera-Resistant  Stocks.  376. 

334.  Preliminary  Volume  Tables  for  Second- 

Growth  Redwood.  377. 

335.  Cocoanut   Meal    as    a    Feed   for   Dairy  3  79. 

Cows  and  Other  Livestock.  380. 

339.  The  Relative  Cost  of  Making  Logs  from 

Small  and  Large  Timber.  381. 

340.  Control  of  the  Pocket  Gopher  in  Cali- 

fornia. 382. 

343.  Cheese  Pests  and  Their  Control. 

344.  Cold  Storage  as  an  Aid  to  the  Market-  383. 

ing  of  Plums. 

346.  Almond  Pollination.  384. 

347.  The  Control  of  Red  Spiders  in  Decidu- 

ous Orchards. 

348.  Pruning  Young  Olive  Trees.  385. 

349.  A    Study    of     Sidedraft    and    Tractor  386. 

Hitches. 

350.  Agriculture  in  Cut-over  Redwood  Lands. 


Further  Experiments  in  Plum  Pollina- 
tion. 

Bovine  Infectious  Abortion. 

Results  of  Rice  Experiments  in   1922. 

A  Self-mixing  Dusting  Machine  for 
Applying  Dry  Insecticides  and 
Fungicides. 

Black  Measles,  Water  Berries,  and 
Related  Vine  Troubles. 

Fruit  Beverage  Investigations. 

Preliminary  Yield  Tables  for  Second 
Growth  Redwood. 

Dust  and  the  Tractor  Engine. 

The  Pruning  of  Citrus  Trees  in  Cali- 
fornia. 

Fungicidal  Dusts  for  the  Control  of 
Bunt. 

Turkish  Tobacco  Culture,  Curing  and 
Marketing. 

Methods  of  Harvesting  and  Irrigation 
in  Relation  to  Mouldy  Walnuts. 

Bacterial  Decomposition  of  Olives  dur- 
ing Pickling. 

Comparison  of  Woods  for  Butter  Boxes. 

Browning  of  Yellow  Newtown  Apples. 

The  Relative  Cost  of  Yarding  Small 
and  Large  Timber. 

The  Cost  of  Producing  Market  Milk  and 
Butterfat  on  246  California  Dairies. 

A  Survey  of  Orchard  Practices  in  the 
Citrus  Industry  of  Southern  Cali- 
fornia. 

Results  of  Rice  Experiments  at  Cor- 
tena,    1923. 

Sun-Drying  and  Dehydration  of  Wal- 
nuts. 

The  Cold  Storage  of  Pears. 

Walnut  Culture  in  California. 

Growth  of  Eucalyptus  in  California 
Plantations. 

Growing  and  Handling  Asparagus 
Crowns. 

Pumping  for  Drainage  in  the  San 
Joaquin  Valley,   California. 

Monilia  Blossom  Blight  (Brown  Rot) 
of  Apricot. 

A  Study  of  the  Relative  Values  of  Cer- 
tain Succulent  Feeds  and  Alfalfa  Meal 
as  Sourses  of  Vitamin  A  for  Poultry. 

Pollination  of  the  Sweet  Cherry. 

Pruning  Bearing  Deciduous  Fruit 
Trees. 


CIRCULARS 
No.  No. 

87.   Alfalfa.  155. 

113.   Correspondence  Courses  in  Agriculture.  157. 

117.  The    Selection    and    Cost    of    a    Small  160. 

Pumping  Plant.  161. 

127.   House  Fumigation.  164. 

129.   The  Control  of  Citrus  Insects.  165. 

136.  Melilotus    indica    as    a    Green-Manure 

Crop  for  California. 
144.    Oidium  or  Powdery  Mildew  of  the  Vine. 

151.  Feeding  and  Management  of  Hogs. 

152.  Some  Observations  on  the  Bulk  Hand- 

ling of  Grain   in   California. 
154.   Irrigation   Practice   in   Growing   Small 
Fruit  in  California. 


166. 
167. 
170. 

173. 

178. 


Bovine  Tuberculosis. 

Control  of  the  Pear  Scab. 

Lettuce  Growing  in  California. 

Potatoes  in  California. 

Small  Fruit  Culture  in  California. 

Fundamentals   of    Sugar    Beet    Culture 

under  California  Conditions. 
The  County  Farm   Bureau. 
Feeding  Stvffs  of  Minor  Importance. 
Fertilizing  California  Soils  for  the  1918 

Crop. 
The    Construction    of    the    Wood-Hoop 

Silo. 
The  Packing  of  Apples  in  California. 


CIRCULARS—  ( Continued ) 


No.  No. 

179.   Factors    of    Importance    in    Producing  251. 

Milk  of  Low  Bacterial  Count. 
184.  A  Flock  of  Sheep  on  the  Farm. 
190.   Agriculture  Clubs  in  California.  252. 

199.   Onion  Growing  in  California.  253. 

202.  County   Organizations   for   Rural   Fire  254. 

Control. 

203.  Peat  as  a  Manure  Substitute.  255. 

208.  Summary  of  the  Annual  Reports  of  the 

Farm  Advisors  of  California.  256. 

209.  The  Function  of  the  Farm  Bureau.  257. 

210.  Suggestions 'to  the  Settler  in  California.  258. 
212.   Salvaging  Rain-Damaged  Prunes.  259. 

214.  Seed  Treatment  for  the  Prevention  of  260. 

Cereal  Smuts. 

215.  Feeding  Dairy  Cows  in  California.  261. 
217.  Methods   for   Marketing  Vegetables   in  262. 

California.  263. 

220.   Unfermented  Fruit  Juices.  264. 

228.   Vineyard  Irrigation  in  Arid  Climates. 

231.  The  Home  Vineyard.  265! 

232.  Harvesting    and    Handling    California  266. 

Cherries  for  Eastern   Shipment. 

233.  Artificial  Incubation.  267. 

234.  Winter  Injury  to  Young  Walnut  Trees 

during  1921-22.  268. 

235.  Soil  Analysis  and  Soil  and  Plant  Inter- 

relations. 269. 

236.  The  Common  Hawks  and  Owls  of  Cali-  270. 

fornia    from    the    Standpoint   of    the  271. 

Rancher.  272. 

237.  Directions  for  the  Tanning  and  Dress- 

of  Furs.  273. 

238.  The  Apricot  in  California.  275. 

239.  Harvesting  and  Handling  Apricots  and 

Plums  for  Eastern  Shipment.  276. 

240.  Harvesting    and    Handling    Pears   for  277. 

Eastern  Shipment. 

241.  Harvesting  and  Handling  Peaches  for  278. 

Eastern  Shipment.' 

243.  Marmalade  Juice  and  Jelly  Juice  from  279. 

Citrus  Fruits. 

244.  Central  Wire  Bracing  for  Fruit  Trees.  281. 

245.  Vine  Pruning  Systems. 

247.  Colonization  and  Rural  Development. 

248.  Some  Common  Errors  in  Vine  Pruning  282. 

and  Their  Remedies. 

249.  Replacing  Missing  Vines. 

250.  Measurement   of   Irrigation   Water   on 

the  Farm. 


Recommendations  Concerning  the  Com- 
mon Diseases  and  Parasites  of 
Poultry  in  California. 

Supports  for  Vines. 

Vineyard  Plans. 

The  Use  of  Artificial  Light  to  Increase 
Winter  Egg  Production. 

Leguminous  Plants  as  Organic  Fertil- 
izer in  California  Agriculture. 

The  Control  of  Wild  Morning  Glory. 

The  Small-Seeded  Horse  Bean. 

Thinning  Deciduous  Fruits. 

Pear  By-products. 

A  Selected  List  of  References  Relating 
to  Irrigation  in  California. 

Sewing  Grain   Sacks. 

Cabbage  Growing  in  California. 

Tomato  Production  in  California. 

Preliminary  Essentials  to  Bovine  Tuber- 
culosis Control. 

Plant  Disease  and  Pest  Control. 

Analyzing  the  Citrus  Orchard  by  Means 
of  Simple  Tree  Records. 

The  Tendency  of  Tractors  to  Rise  in 
Front;  Causes  and  Remedies. 

Inexpensive  Lavor-saving  Poultry  Ap- 
pliances. 

An  Orchard  Brush  Burner. 

A  Farm  Septic  Tank. 

Brooding  Chicks  Artificially. 

California  Farm  Tenancy  and  Methods 
of  Leasing. 

Saving  the  Gophered  Citrus  Tree. 

Marketable  California  Decorative 
Greens. 

Home  Canning. 

Head,  Cane,  and  Cordon  Pruning  of 
Vines. 

Olive  Pickling  in  Mediterranean  Coun- 
tries. 

The  Preparation  and  Refining  of  Olive 
Oil  in  Southern  Europe. 

The  Results  of  a  Survey  to  Determine 
the  Cost  of  Producing  Beef  in  Cali- 
fornia. 

Prevention  of  Insect  Attack  on  Stored 
Grain. 


The  publications  listed  above  may  be  had  by  addressing 

College  of  Agriculture, 

University  of  California, 

Berkeley,  California. 


25m-4,'25 


